1. Field of the Invention
This invention relates to an improvement in a two-cylinder V-type spark-ignition engine with overhead camshafts, generally called an overhead (OHC) V-2 engine.
2. Description of the Related Art
Various OHC V-2 engines are known and one example of such known OHC V-2 engines is disclosed in Japanese Patent Laid-open Publication No. SHO-58-167875. The disclosed engine has two cylinders mounted to a crankcase such that the cylinders are arranged at an angle to each other in the longitudinal direction of a vehicle body on which the engine is installed. The engine also has a valve mechanism having a camshaft mounted in a cylinder head of each cylinder. The cylinder head is bolted to an upper end of a cylinder block to thereby form a single cylinder
Each of the cylinders has an intake port and an exhaust port. The intake port opens to one surface of the cylinder which faces toward another cylinder, and the exhaust port opens to the opposite surface of the cylinder which faces away from another cylinder. With this arrangement of the intake and exhaust ports, two intake pipes connected at one end to the respective intake ports and carburetors associated with the respective intake pipes are all disposed in a relatively small space defined between the two cylinders. This poses a limited degree of freedom in arranging the intake pipes, tending to hinder uniform and efficient supply of fresh air to the cylinders.
Furthermore, in the conventional OHC V-2 engine, since a power transmitting mechanism for transmitting power from a crankshaft to the camshaft of each cylinder for driving the latter is disposed on both left and right sides of the crankcase adjacent to opposite longitudinal ends of the crankshaft, the overall width and weight of the engine are relatively large. In addition, due to the aforesaid arrangement, the power transmitting mechanisms require the engine to be turned through an angle of 180xc2x0 during assembly or maintenance of the power transmitting mechanisms. This may lower the productivity of the engine.
Furthermore, since the crankcase has a partition wall disposed between a driving gear of each power transmitting mechanism and an adjacent connecting rod of the crankshaft, the width of the crankcase is relatively large. In addition, the power transmitting mechanisms are of the two-stage transmission type including an intermediate shaft carrying thereon a first intermediate gear held in mesh with a driving gear attached to the crankcase and a second intermediate gear connected by a chain to a driven gear attach to each of the camshafts. The power transmitting mechanisms are relatively large in size, require a relatively large number of components, are expensive to manufacture, and enlarge the overall size of the engine.
It is accordingly an object of the present invention to provide an overhead camshaft V-2 engine having structural features which enable downsizing of the engine, provide an increased degree of freedom in arranging intake pipes, facilitate easy assembly and maintenance of the engine, and enable efficient lubrication of valve mechanisms with a compact lubricating mechanism.
According to the present invention, there is provided an overhead camshaft V-2 engine comprising: a crankcase; a crankshaft rotatably supported in the crankcase; two cylinder blocks each having one cylinder head and mounted to the crankcase such that the cylinder blocks are arranged at an angle to each other; two valve mechanisms each mounted to the cylinder head of one of the cylinder blocks and having one camshaft; a single power transmitting mechanism disposed on one side of the crankcase for transmitting rotational power from the crankshaft to the camshafts to drive the valve mechanisms, the power transmitting mechanism having a driving member mounted to only one end portion of the crankshaft; and two intake ports formed in the respective cylinder heads of the cylinder blocks and opening at one end to respective first surfaces of the cylinder heads facing in a first direction, and two exhaust ports formed in the respective cylinder heads of the cylinder blocks and opening at one end to respective second surfaces of the cylinder heads facing in a second direction opposite to the first direction. The engine may further include a cooling fan mounted to the opposite end portion of the crankshaft.
The power transmitting mechanism thus arranged occupies only a relatively small space for installation and hence can reduce the overall size of the engine. In addition, the foregoing arrangement of the intake and exhaust ports enables intake pipes to be arranged with a high degree of freedom.
In one preferred form of the invention, the driving member of the power transmitting mechanism includes a pair of driving pulleys mounted in tandem on the crankshaft. It is preferable that the pitch or distance between the pair of driving pulleys is substantially equal to the pitch or distance between two connecting rods attached side by side to a single offset crankpin of the crankshaft.
The power transmitting mechanism may further comprise a pair of driven pulleys connected to the respective camshafts of the valve mechanisms, and a pair of driving belts each trained around one of the driving pulleys and a corresponding one of the driven pulleys.
Preferably, the crankshaft has a boss of an enlarged diameter held in abutment with an end face of one of the driving pulleys and slidably engageable with an outside edge of one of the driving belts for guiding the one driving belt, and the engine further has a ring-like belt guide mounted on the crankshaft and disposed between an end face of the other driving pulley and a portion of the crankcase, the belt guide being slidably engageable with an outside edge of the other driving belt for guiding the other belt.
The overhead camshaft V-2 engine may further comprise a centrifugal governor mechanism disposed in the crankcase for controlling rotational speed of the engine, and a lubricating mechanism for lubricating movable parts of the engine. The governor mechanism preferably has a generally cup-shaped holder rotatably driven by the crankshaft and being partly dipped below a lubricating oil held at a bottom of the crankcase. The lubricating mechanism preferably has a plurality of circumferentially spaced oil splashing projections formed on an outer peripheral surface of the cup-shaped holder so that when the cup-shaped holder is rotating by the rotational power of the crankshaft, the oil splashing projections continuously revolve about an axis of rotation of the cup-shaped holder to thereby draw up the lubricating oil from the bottom of the crankcase and splash the lubricating oil over at least a part of the power transmitting mechanism. The driving chains of the power transmitting mechanism form part of the lubricating mechanism and is capable of guiding the lubricating oil into the valve mechanisms. The governor mechanism may further have a driving gear formed integrally with the driving member and disposed between the pair of driving pulleys, the driving gear being connected in driving relation to the cup-shaped holder for rotating the holder.
The above and other object, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which preferred structural embodiments incorporating the principles of the invention are shown by way of illustrative examples.